High-resolution X-ray spectroscopy of Cen X−3 with XMM-Newton

¹ Department of Physics, Systems Engineering and Signal Theory, University of Alicante, 03080 Alicante, Spain
² University Institute of Physics Applied to Sciences and Technologies, University of Alicante, 03080 Alicante, Spain

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Received: 30 December 2024
Accepted: 25 January 2026

Abstract

Aims. The spectral analysis of two XMM-Newton observations of the high-mass X-ray binary system Cen X−3 is presented. In particular, it is focused on the eclipse and out-of-eclipse spectra in order to compare the properties of the environment around the compact object.

Methods. The high-resolution spectra obtained from the reflection grating spectrometer on board XMM-Newton were analysed, with a focus on studying eclipse and out-of-eclipse spectra separately. Several continuum models were explored in SPEX, for which we studied the properties of emitting and absorbing matter depending on the emission and absorption lines identified in the spectra.

Results. It was found that the X-ray continuum is heavily absorbed by a neutral gas and photoionised matter. Emission lines from Si V, Mg XII, Mg XI, and Ne X were detected in the eclipse spectrum; in particular, H-like lines of Mg and Ne with a significance of ≳5σ in the eclipse spectrum and ∼3σ in the out-of-eclipse spectrum. However, in the out-of-eclipse spectrum any absorption lines were detected with a significance of less than 2σ. RGS light curve showed dips in the out-of-eclipse spectrum that are not due to an increase in the column absorption but that may be produced by instabilities in the accretion stream. On the other hand, the level of counts above 20 Å was compatible with the X-ray background. A simple local continuum model was used to describe the He-like triplet of Ne and the derived values of ℛ and 𝒢 ratio parameters pointed out that the UV photospheric field should be important at the line production site and an electron density greater than 10¹² cm⁻³. As a consequence, a hybrid plasma may be present in the binary system.